Comparison Between Vacuum Circuit Breaker and SF6 Circuit Breaker

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Vacuum Circuit Breaker

In a Vacuum circuit breaker, vacuum interrupters are used for breaking and making load and fault currents. When the contacts in vacuum interrupter separate, the current to be interrupted initiates a metal vapour arc discharge and flows through the plasma until the next current zero.

The arc is then extinguished and the conductive metal vapour condenses on the metal surfaces within a matter of micro seconds. As a result the dielectric strength in the breaker builds up very rapidly.

SF6 Gas Circuit Breaker

In an SF6 circuit-breaker, the current continues to flow after contact separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally de-ionises the contact gap and establishes the dielectric strength required to prevent a re-strike.

Table 1. Characteristics of the SF6 and vacuum current interrupting technologies.


SF6 Circuit Breakers

Vacuum Circuit Breakers

Criteria

Puffer Circuit Breaker

Self-pressuring circuit-breaker

Contact material-Chrome-Copper

Operating energy requirements

Operating Energy requirements are high, because the mechanism must supply the energy needed to compress the gas.

Operating Energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over short distances. The mechanism does not have to provide the energy to create the gas flow

Operating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances.

Arc Energy

Because of the high conductivity of the arc in the SF6 gas, the arc energy is low. (arc voltage is between 150 and 200V.)

Because of the very low voltage across the metal vapour arc, energy is very low. (Arc voltage is between 50 and 100V.)

Contact Erosion

Due to the low energy the contact erosion is small.

Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapour re-condenses on the contact, contact erosion is extremely small.

Arc extinguishing media

The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. The gas pressure can be very simply and permanently supervised. This function is not needed where the interrupters are sealed for life.

No additional extinguishing medium is required. A vacuum at a pressure of 10-7 bar or less is an almost ideal extinguishing medium. The interrupters are ‘sealed for life’ so that supervision of the vacuum is not required.

Switching behavior in relation to current chopping

The pressure build-up and therefore the flow of gas is independent of the value of the current. Large or small currents are cooled with the same intensity. Only small values of high frequency, transient currents, if any, will be interrupted. The de-ionization of the contact gap proceeds very rapidly, due to the electro-negative characteristic of the SF6 gas and the arc products.

The pressure build-up and therefore the flow of gas is dependent upon the value of the current to be interrupted. Large currents are cooled intensely, small currents gently. High frequency transient currents will not, in general, be interrupted. The de-ionization of the contact gap proceeds very rapidly due to the electro-negative characteristic of the SF6 gas and the products.

No flow of an ‘extinguishing’ medium needed to extinguish the vacuum arc. An extremely rapid de-ionization of the contact gap, ensures the interruption of all currents whether large or small. High frequency transient currents can be interrupted. The value of the chopped current is determined by the type of contact material used. The presence of chrome in the contact alloy with vacuum also.

No. of short-circuit operation

10—50

10—50

30—100

No. full load operation

5000—10000

5000—10000

10000—20000

No. of mechanical operation

5000—20000

5000—20000

10000—30000

 

Table2. Comparison of the SF6 And Vacuum Technologies In Relation To Operational Aspects

Criteria

SF6 Breaker

Vacuum Circuit Breaker

Summated current cumulative

10-50 times rated short circuit current

30-100 times rated short circuit current

Breaking current capacity of interrupter

5000-10000 times

10000-20000 times

Mechanical operating life

5000-20000 C-O operations

10000-30000 C-O operations

No operation before maintenance

5000-20000 C-O operations

10000-30000 C-O operations

Time interval between servicing Mechanism

5-10 years

5-10 years

Outlay for maintenance

Labour cost High, Material cost Low

Labour cost Low, Material cost High

Reliability

High

High

Dielectric withstand strength of the contact gap

High

Very high

Table3. Comparison of the SF6 And Vacuum Switching Technologies In Relation To Switching Applications

Criteria

SF6 Circuit Breaker

Vacuum Circuit Breaker

Switching of Short circuit current with High DC component

Well suited

Well suited

Switching of Short circuit current with High RRV

Well suited under certain conditions (RRV>1-2 kV per Milli seconds

Very well suited

Switching of transformers

Well suited.

Well suited

Switching of reactors

Well suited

Well suited. Steps to be taken when current <600A. to avoid over voltage due to current chopping

Switching of capacitors

Well suited. Re-strike free

Well suited. Re-strike free

Switching of capacitors back to back

Suited. In some cases current limiting reactors required to limit inrush current

Suited. In some cases current limiting reactors required to limit inrush current

Switching of arc furnace

Suitable for limited operation

Well suited. Steps to be taken to limit over voltage.


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